well depending on the age of the engine, the engine(cfm 56-5) should be allowed between 3mins(new engine)-5mins(older engine) to warm up to normal temperature before going to TO power. failure to do so will shorten engine life and increase maintenance costs. some engine specialists that i have spoken too, do not recommend 1 engine taxi because pilots do not allow enough time for warm up of the other engine.... so please be nice to the engines and don't worry too much about saving fuel in this case